JWST discovers ancient galaxy cluster with unexpected dark-matter concentration

XLSSC 122, an ancient galaxy cluster seen at a time when such massive structures were just beginning to assemble, shows a surprisingly high concentration of mass in its center, contradicting standard cosmological predictions. JWST’s imaging captured strong gravitational lensing arcs, confirming the cluster’s ability to bend light from more distant galaxies and providing a precise probe of its dark-matter content. Complementary weak-lensing analysis and multi-wavelength observations indicate the cluster is undergoing a merger, while the detection of faint intracluster light suggests stars stripped from galaxies trace the dark-matter layout.

The findings, presented at the American Astronomical Society meeting, stem from three papers led by IPAC scientists Kyle Finner, Zachary Scofield and Hyungjin Joo. The discovery highlights the potential of JWST to test models of structure formation in the early universe.